This invention relates to modulation systems and methods and more particularly to systems and methods that can efficiently modulate a signal onto a carrier.
Modulation systems and methods are widely used in transmitters to modulate information including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as xe2x80x9cmixersxe2x80x9d or xe2x80x9cmultipliersxe2x80x9d. For example, in a mobile radiotelephone, a modulator is used in the radiotelephone transmitter.
In modern radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to provide modulation systems and methods that can provide high power modulation while reducing the amount of battery power that is consumed. Unfortunately, the power amplifier of a modulator may consume excessive power due to efficiency limitations therein. More specifically, it is known to provide linear Class-A or Class-AB power amplifiers that may have efficiencies as low as 30 percent or less. Thus, large amounts of battery power may be wasted as heat.
A major breakthrough in high efficiency modulation systems and methods is described in application Ser. No. 09/195,384 to the present co-inventor Camp, Jr., et al., entitled Circuit and Method for Linearizing Amplitude Modulation in a Power Amplifier (now U.S. Pat. No. 6,191,653); application Ser. No. 09/195,129 to the present co-inventor Camp, Jr., et al., entitled Circuit and Method for I/Q Modulation with Independent, High Efficiency Amplitude Modulation; application Ser. No. 09/207,167 to the present co-inventor Camp., Jr., entitled Amplitude Modulation to Phase Modulation Cancellation Method in an RF Amplifier (now U.S. Pat. No. 6,295,442) and application Ser. No. 09/226,478 to the present co-inventor Camp, Jr., et al., entitled Power I/Q Modulation Systems and Methods (now U.S. Pat. No. 6,181,199). Each of these copending applications is assigned to the assignee of the present application and the disclosures of all these applications are hereby incorporated herein by reference. These applications describe systems and methods for modulating phase and amplitude separately using high efficiency amplifiers.
FIG. 1 is a block diagram of power modulation systems and methods that separately amplify amplitude and phase according to the above-cited copending applications. As shown in FIG. 1, these power modulation systems and methods 100 include a waveform generator 102 that generates an amplitude waveform G(A(t)) 104 and a phase waveform F(xcfx86(t)) 106 from a plurality of data symbols 108. A phase modulator 110, such as a voltage controlled oscillator (VCO) and/or a phase locked loop (PLL), phase modulates the phase waveform 106 to produce a phase modulated waveform 112. An RF driver amplifier 114 may be included in order to overdrive a power amplifier 116. The power amplifier 116 is preferably a Class-C power amplifier that includes a power supply input 116a, a signal input 116b, and a power output 116c. The phase modulated waveform 112 is applied to the signal input 116bof the Class-C power amplifier 116, either directly from the phase modulator 110 or via the RF driver amplifier 114.
Continuing with the description of FIG. 1, a Class-D amplifier 120 is responsive to the amplitude waveform 104 to supply a variable supply voltage to the power supply input 116a of the power amplifier 116. An analog-to-digital converter, for example a delta sigma modulator 124 that operates from a clock frequency fcl, converts the amplitude waveform 104 to a digital signal that is applied to the Class-D amplifier 120. The output of the Class-D amplifier 120 is then lowpass filtered, for example at a frequency fc, by a lowpass filter 126. The amplified amplitude waveform 122 is applied to the power supply input 116a of the power amplifier 116, and the phase modulated waveform 112 is applied to the signal input 116b of the power amplifier 116, to produce a power modulated waveform 130 of the data symbols 108 at the power output 116c. Thus, power modulation systems and methods of FIG. 1 control the supply voltage to the Class-C power amplifier 116 using the Class-D power amplifier 120 in order to maintain a high overall efficiency.
As power modulators are used with increasingly higher frequencies, it may be desirable to further extend the frequency response thereof. For example, satellite radiotelephone communication systems may operate at relatively high frequencies compared to terrestrial radiotelephone communication systems. Accordingly, it may be desirable to extend the frequency response of power modulation systems and methods that separately modulate phase and amplitude.
The frequency response of the power modulation systems and methods of FIG. 1 may be extended by increasing the cutoff frequency fc of the lowpass filter 126. Unfortunately, if the cutoff frequency of the lowpass filter is increased, noise from the delta sigma modulator 124 may be imparted on the power modulated waveform. Alternatively, a higher clock frequency fcl may be used for the delta sigma modulator. Unfortunately, this may increase the cost and/or decrease the efficiency of the delta sigma modulator 124 and/or the Class-D amplifier 120. Accordingly, there continues to be a need for power modulation systems and methods that can separately modulate phase and amplitude at high frequencies.
It is therefore an object of the present invention to provide improved power modulation systems and methods.
It is another object of the invention to provide improved power modulation systems and methods that can separately modulate amplitude and phase.
It is yet another object of the present invention to provide systems and methods that can extend the frequency response of power modulation systems and methods that separately modulate amplitude and phase.
These and other objects are provided, according to the present invention by separately amplifying a low frequency portion of an amplitude waveform and a high frequency portion of the amplitude waveform. The low frequency portion, generally containing most of the modulation energy, is preferably amplified at high efficiency to produce an amplified low frequency portion, while the high frequency portion may be amplified at lower efficiency to produce an amplified high frequency portion. The amplified low frequency portion and the amplified high frequency portion are combined to produce a combined amplified amplitude waveform that is applied to the power supply input of a high efficiency power amplifier.
The invention stems from the realization that the energy versus frequency in a conventional amplitude modulation spectrum generally diminishes rapidly with increasing frequency. Accordingly, efficient modulation is used for the low frequency portion of the amplitude waveform. Amplitude modulation frequency components above that frequency may be supplied using more conventional linear methods of modulation and amplification. Thus, the bulk of the amplitude modulation power may be supplied via efficient amplification, while the remaining frequency components, representing very little power, may be supplied via a less efficient but more convenient conventional linear control circuit. Overall high efficiency therefore may be maintained while allowing an extended bandwidth signal to be modulated.
More specifically, power modulation systems and methods according to the invention generate an amplitude waveform and phase waveform from a plurality of data symbols. The amplitude waveform includes a low frequency portion and a high frequency portion. The phase waveform is phase modulated to produce a phase modulated waveform. A power amplifier, preferably a Class-C power amplifier, includes a power supply input, a signal input and a power output. The phase modulated waveform is applied to the signal input. The low frequency portion of the amplitude waveform is amplified at high efficiency to produce an amplified low frequency portion. The high frequency portion is amplified at lower efficiency to produce an amplified high frequency portion. The amplified low frequency portion and the amplified high frequency portion are combined to produce a combined amplified amplitude waveform. The combined amplified amplitude waveform is applied to the power supply input of the power amplifier to produce a power modulated waveform of the data symbols at the power output of the power amplifier.
In a preferred embodiment of the present invention, highpass filtering of the amplitude waveform is used to generate the high frequency portion. The amplifier for the low frequency portion includes a lowpass filter therein. The lowpass filter and the highpass filter have the same cutoff frequency so that flat frequency response is obtained. In another embodiment, the lowpass filter cutoff frequency may vary due to manufacturing tolerance variations and/or other causes. In order to compensate for the variable lowpass filtering, the highpass filter may have an adjustable highpass cutoff frequency that may be adjusted to be the same as the lowpass cutoff frequency. In one embodiment, the lowpass cutoff frequency may be determined using test tones or other probing techniques. The adjustable highpass cutoff frequency is then adjusted to be the same as the lowpass cutoff frequency that is so determined. In another embodiment, the amplitude waveform may be lowpass filtered at a first lowpass cutoff frequency that is lower than the lowpass cutoff frequency that is included in the amplifier for the low frequency portion. This first lowpass cutoff frequency can be accurately controlled, for example by using a digital lowpass filter, so that the highpass filter also may be accurately matched to the digital lowpass filter.
In other embodiments of the present invention, prefiltering may be used to extend the frequency response of the low frequency amplifier. In particular, the low frequency amplifier includes a lowpass filter therein having a lowpass cutoff frequency. The amplitude of the amplitude waveform is raised above the lowpass cutoff frequency to thereby extend the lowpass cutoff frequency to a second lowpass cutoff frequency that is higher than the lowpass cutoff frequency. The amplitude waveform is highpass filtered at the second lowpass cutoff frequency to thereby generate the high frequency portion. An adjustable highpass filter may be used and adjusted to be the same as the second lowpass cutoff frequency.
In yet another embodiment of the present invention, the amplitude waveform may be prefiltered to raise the amplitude of the amplitude waveform above the lowpass cutoff frequency according to a frequency response that is complimentary to the lowpass filter, to thereby flatten the response of the low frequency amplifier beyond the lowpass cutoff frequency. More specifically, the amplitude waveform is prefiltered to produce a prefiltered amplitude waveform. The prefiltered amplitude waveform is amplified to produce an amplified prefiltered amplitude waveform. The amplifier includes a lowpass filter therein having a first cutoff frequency. The prefiltering raises the amplitude of the amplitude waveform above the first lowpass cutoff frequency to thereby extend the first lowpass cutoff frequency to a second lowpass cutoff frequency that is higher than the first lowpass cutoff frequency. More preferably, the amplitude of the amplitude waveform is raised above the first lowpass cutoff frequency according to a frequency response that is complimentary to the lowpass filter to thereby flatten the response of the low frequency amplifier beyond the first lowpass cutoff frequency. In this case, the portion of the amplitude waveform that is amplified by the high frequency lower efficiency amplifier can be reduced or eliminated.
In all of the above embodiments, the low frequency portion preferably is amplified in a Class-D amplifier, the high frequency portion preferably is amplified in a linear amplifier, and the power amplifier is a Class-C amplifier. A delta sigma modulator may be coupled between the waveform generator and the Class-D amplifier to provide analog-to-digital conversion. According to another aspect, the low frequency amplifier produces nonlinear distortion and the power modulation systems and methods generate a predistorted amplitude waveform from the amplitude waveform so that the distorted amplitude waveform is amplified to thereby compensate for the nonlinear distortion.
Accordingly, the frequency response of high efficiency power modulation systems and methods may be extended by amplifying high frequencies using conventional amplifiers. Since the bulk of the amplitude modulation power is supplied via efficient power amplification, high efficiency still may be obtained. Thus, the frequency response may be extended without the need to provide more costly high frequency delta sigma modulators and/or Class-D amplifiers and without the need to introduce additional noise into the modulated waveform. High performance high efficiency power modulation systems and methods thereby may be provided with extended frequency response.